Specific energy and net specific energy of energy storage devices.
Lithium‐ion capacitors (LICs) offer higher energy density and specific energy than do traditional electric double‐layer capacitors (EDLCs). In spacecraft power systems where traditional lithium‐ion batteries (LIBs) have been used with shallow depth of discharge (DoD) in order to achieve long‐cycle life, LICs would potentially be an alternative to secondary batteries. Firstly, this chapter presents the quantitative comparison between the LIB‐ and LIC‐based spacecraft power system from the viewpoint of system mass. On the basis of the potential suggested by the comparison, we have been developing the technology demonstration platform named “NESSIE” that contains the LIC pouch cell as one of its major demonstration missions. NESSIE was successfully launched with the main satellite HISAKI on September 2013. This chapter also presents the development of the LIC pouch cell for NESSIE and its experimental (or ground test) and on‐orbit operation data.
- cycle life testing
- lithium‐ion capacitor
- pouch cell
- spacecraft power system
- vacuum tolerance
Applications of secondary battery‐based energy storage are rapidly expanding from portable electronic devices to large‐scale systems, including electric vehicles and grid‐connected applications. Among various secondary battery chemistries, lithium‐ion batteries (LIBs) are the most promising and viable for portable and vehicular applications thanks to their highest energy density and specific energy. Vigorous research and development efforts for increased energy density and extended service life are underway.
Meanwhile, supercapacitors (SCs), formally known as electric double‐layer capacitors (EDLCs), are also an attractive energy storage device that plays an important role in various scenes. Thanks to their energy storage mechanism utilizing the double‐layer capacitance on porous‐activated carbon electrodes, EDLCs offer remarkable advantages over traditional secondary batteries in terms of cycle life, power capability, safety, and allowable temperature range. However, their low‐energy density property is generally considered as a major drawback to be used as an alternative energy storage source to traditional secondary batteries. Hence, applications of EDLCs are chiefly limited to hybrid energy storage applications where EDLCs play a role of the high‐power energy buffer that complements the main energy sources, such as secondary batteries and fuel cells.
To cope with the drawback of the low‐energy density of EDLCs, lithium‐ion capacitors (LICs) have been developed and commercialized by several manufacturers. LICs are basically a hybrid electrochemical capacitor combining double‐layer capacitance and lithium intercalation for energy storage mechanism, and they offer the higher specific energy than EDLC without sacrificing the major benefits of EDLCs. LICs have been drawing significant attentions in vehicular and industry applications where high‐power energy buffers are indispensable. Despite the increased energy density property, there is still a huge gap between LICs and secondary batteries.
However, once the excellent long‐cycle life performance of EDLCs and LICs is factored in, it drives expectation that EDLCs and LICs would be an alternative energy storage source to traditional secondary batteries in certain applications. For example, EDLC‐based uninterruptible power supplies (UPSs) have been commercialized , as shown in Figure 1, as maintenance‐free alternatives aiming for infrastructure applications where traditional lead‐acid batteries need to be replaced with new ones for every few years. Another potential application is a spacecraft power system where long‐life energy storage sources are essential. In the previous study [2, 3], systematic cycle life testing emulating low‐Earth‐orbit (LEO) satellite conditions has been performed for EDLCs and LICs, and the results demonstrated that excellent cycle life performance can be expected for cycling conditions for LEO satellites.
For spacecraft applications where specific energy is of great importance, LICs are undoubtedly superior to EDLCs. Despite the great potential suggested in the laboratory cycle life testing, LICs had not been used in practical space applications. In order to demonstrate the LIC performance in practical on‐orbit conditions, we developed the technology demonstration platform named ‘NESSIE (NExt‐generation Small Satellite Instrument for Electric power systems)’ containing an LIC pouch cell as one of its major components to be demonstrated on orbit; NESSIE’s major missions are (1) on‐orbit demonstration for the LIC cells and (2) on‐orbit demonstration for high‐efficiency thin‐film solar cells. NESSIE was embedded in the main satellite named “HISAKI”, the Spectroscopic Planet Observatory for Recognition of Interaction of Atmosphere (see Figure 2) and was launched on September 14, 2013.
In this chapter, the quantitative comparison between LIB‐ and LIC‐based spacecraft power systems is performed in terms of system mass. This chapter also presents the development of the LIC pouch cell for NESSIE and its experimental (or ground test) and on‐orbit operation data.
2. Cycle life of LICs
In the previous study , cycle life testing emulating LEO spacecraft conditions was performed for LICs, and capacitance retentions of LICs were compared to those of LIBs under the same temperature conditions, as shown in Figure 3. The capacitance retention of the LIB with the shallow DoD of 20% dropped to approximately 70% at 10,000 cycles, which is equivalent to approximately 2‐year operation, while that of LICs was still greater than 95%. Furthermore, different from LIBs whose retention trends were dependent on DoD, the LICs showed DoD‐independent trends.
The accelerated ageing testing and the cycle life prediction model for LICs were also investigated in the previous study . LIC cells were cycled at various temperatures in the systematically designed cycle test matrix. The activation energies of degradation ratios of LIC cells were calculated using the Arrhenius equation, whereupon ageing acceleration factors were determined. The experimental and predicted capacitance retention trends matched satisfactorily, and hence, the established cycle life prediction model was verified. According to the reported results in , LICs are expected to operate even longer than millions of cycles at temperature lower than 10°C.
The mission life of the main satellite HISAKI was 1 year, and therefore, the cycle life of LICs reported in the previous studies was adequate for NESSIE. However, the cycle life testing reported in the previous study was performed under laboratory conditions, leaving the investigation of tolerance against space environment, such as vacuum, vibration, and radiation, as issues to be cleared. Section 4 discusses these issues in detail.
3. LIC‐ and LIB‐based power system comparison
3.1. Specific energy
Cycle life performance of traditional secondary batteries is greatly dependent on DoD. In general, secondary batteries for LEO satellites are operated with DoD shallower than 40% in order to meet the typical cycle life requirement of 30,000 cycles that is equivalent to approximately 5.7 years on‐orbit operation. In other words, the net specific energy of secondary batteries is far lower than their rated specific energy because of the shallow DoD operation. Specific energies as well as net specific energies of various energy storage devices are compared in Table 1. Although LIBs offer the highest specific energy of 150 Wh/kg, their net specific energy drops to as low as 60 Wh/kg with DoD of <40%.
|Alkaline battery (Ni‐Cd,|
Ni‐MH, and Ni‐H2)
|Specific energy||40–60 Wh/kg||150 Wh/kg||<10 Wh/kg||<30 Wh/kg|
|Depth of discharge||<40%||<40%||<80%||<80%|
|Net specific energy||<24 Wh/kg||<60 Wh/kg||<8 Wh/kg||<24 Wh/kg|
EDLCs and LICs, on the other hand, offer excellent cycle life performance that is insensitive to DoD conditions, allowing deep DoD operations. In the case of the 80% DoD operation for EDLCs and LICs, for example, their net specific energies are <8 Wh/kg and <24 Wh/kg, respectively, greatly bridging the gap in terms of net specific energy.
3.2. Photovoltaic array reduction by constant power charging scheme
A charging profiles of a 10‐Wh LIB cell with a constant‐current‐constant‐voltage (CC‐CV) charging scheme are shown in Figure 4(a) as typical characteristics. The charging power peaks at the end of the CC charging period (or at the beginning of the CV charging period), when both the current and voltage become maximum. The chargeable state of charge (SoC) in the CC charging period is merely 60%, and the rest 40% is charged with CV charging. This characteristic indicates that the CV charging is indispensable for LIBs to reach high SoC. In typical LEO satellites, photovoltaic (PV) arrays are designed to be capable of supplying not only load power but also this peak charging power.
Different from LIBs mentioned above, LICs and EDLCs can be charged to nearly 100% SoC without CV charging because of their low‐impedance properties. A charging profiles of a 10‐Wh LIC cell with the constant‐power (CP) charging scheme, instead of CC‐CV charging scheme, are shown in Figure 4(b). The charging power with the CP charging scheme is constant, and its peak value is substantially lower than that with the CC‐CV charging scheme. This reduction in peak charging power contributes to the reduction in area and mass of PV arrays, as will be exemplified in Section 3.6.
3.3. Power system using LIC
An unregulated bus system using an array power regulator (APR), as shown in Figure 5(a), is mainstream for low‐ to medium‐scale spacecraft power systems using an LIB‐based energy storage. The LIB can be directory connected to the load (or bus) because of the relatively float voltage characteristics. The APR plays a role of battery charging, and a load power is supplied by the LIB and/or APR.
On the other hand, an LIC cannot be directory connected to the bus because of the relatively large‐voltage variation (see Figure 4(b)), and therefore, a charge‐discharge regulator (CDR) is necessary. An LIC‐based power system using a shunt dissipator as a bus voltage regulator is shown in Figure 5(b). In the following subsections, power systems shown in Figure 5 are compared from the viewpoint of mass.
3.4. Mass of LIB‐based power system
A single charge‐discharge cycle consists of the sun and eclipse periods Tsun and Teclipse, respectively. The discharged energy during the eclipse period Edis_LIB is
where Pload is the load power.
Batteries contain not only cells but also mechanical structural support that represents nearly 20% increase in mass over that of cells. Now, the ratio of the mechanical support to cells is defined as A. The mass of the LIB, MLIB, is expressed as
where D is the DoD, and SLIB_cell is the specific energy of LIB cells.
Let Vave be the average voltage of the LIB during discharging. The capacity of the LIB, CLIB, can be determined as
Introducing the charge rate (also known as C‐rate) as Rcha, the charge current for the LIB, Icha, is yielded as
The value of Rcha can be determined from the charge‐discharge ratio that is defined as
Generally, a typical value of RC/D for LEO spacecraft is 1.25 .
The maximum charging power (Pcha_LIB) at the end of the CC charging or at the beginning of the CV charging, at which the LIB voltage is as high as Vcha, is
The mass of the PV arrays in the LIB‐based power system, MPV_LIB, can be determined to be
where ηAPR is the power conversion efficiency of the APR and ρPV is the specific power of the PV array.
The mass of the power conditioning system MPCS_LIB, which is equal to the mass of the APR in the LIB‐based power system, is expressed as
where mAPR is the mass/watt coefficient of the APR.
3.5. Mass of LIC‐based power system
Let ηCDR be the power conversion efficiency of the CDR. The discharged energy of the LIC Edis_LIC can be expressed as
With the specific energy of the LIC cells SLIC_cell, the mass of the LIC, MLIC, can be represented as
Assuming that the LIC is fully charged at the end of the sun period, the power demanded for the PV array from the LIC, Pcha_LIC, is expressed as
The mass of the PV arrays in the LIC‐based power system, MPV_LIC, is
Assuming the shunt dissipator is designed to be capable of the sum of Pload and Pcha_LIC, the mass of the PCS in the LIC‐based power system, MPCS_LIC, is given by
where mCDR and mshunt are the mass/watt coefficient of the CDR and shunt dissipator, respectively.
3.6. System mass comparison
|Load power||Pload||1000 W|
|Sun period||Tsun||1 h|
|Eclipse period||Teclipse||0.5 h|
|C/D ratio (for LIB only)||RC/D||1.25|
|Charge voltage (LIB only)||Vcha||28.7 V|
|Average discharge voltage (LIB only)||Vave||25.9 V|
|Mass ratio of mechanical structural supports to cells||A||20%|
|Specific power of PV array||ρPV||60 W/kg|
|Efficiency of array power regulator||ηAPR||90%|
|Efficiency of charge/discharge regulator||ηCDR||90%|
|Mass/watt coefficient of array power regulator||mAPR||4 kg/kW|
|Mass/watt coefficient of charge/discharge regulator||mCDR||4 kg/kW|
|Mass/watt coefficient of shunt dissipator||mShunt||2 kg/kW|
Figure 6 depicts the system mass of 1‐kW LIB‐ and LIC‐based power systems, with nominal specific energies (SLIB_cell = 150 Wh/kg and SLIC_cell = 30 Wh/kg) ±20% variation, as a function of DoD. As mentioned earlier, in order for LIBs to meet the typical cycle life requirement of 30,000 cycles, DoD should be set as shallow as 40% to mitigate cycling‐induced degradation. The mass of the LIB‐based power system with 40% DoD is around 50 kg and lighter than the LIC‐based system, indicating the traditional LIB‐based power system is superior for the typical 30,000‐cycle life requirement from the viewpoint of system mass. For longer cycle life requirement, on the other hand, LIBs have to be cycled with even shallower DoD, and therefore, the mass of the LIB‐based power system is prone to sharp increase, as can be found around 20% DoD in Figure 6. Hence, the LIB‐based system with shallow DoD competes with the LIC‐based system. The mass of the LIB‐based system with DoD of 15%, for example, is around 70 kg and is comparable with that of the LIC‐based system with 60% DoD.
The mass breakdown of the LIB‐ and LIC‐based power systems with the nominal specific energies of SLIB_cell = 150 Wh/kg and SLIC_cell = 30 Wh/kg are compared in Figure 7. In the LIC‐based power systems, the LIC accounted for greater than half the total mass of the systems. Meanwhile, thanks to the CP charging scheme introduced in Section 3.2, the mass of the PV array in the LIC‐based system can be reduced compared to that in the LIB‐based system. The total mass of the LIC‐based system with DoD of 60–80% is comparable with that of LIB‐based system with DoD of 15–20%, suggesting that LICs would be an alternative energy storage source to LIBs for applications needing long‐cycle life. Other benefits, such as wider operational temperature range of LICs, would further improve the likelihood of LICs being an alternative to traditional LIBs.
4. Development of LIC for NESSIE
4.1. Vacuum tolerance of LIC pouch cell
Needless to say, there is no air in the space, and therefore, components for spacecraft must be vacuum‐tolerant. In general, secondary batteries for spacecraft power systems are reinforced by metal‐housing so as to increase the ruggedness for shock and vibration during launch and tolerance against vacuum in space. In the previous developmental work, electrical characteristics of lithium‐ion pouch cells were investigated . The cells swelled in vacuum, and the performance of LIB pouch cells significantly deteriorated. In order to improve the vacuum tolerance, LIB pouch cells were potted with epoxy resin in an aluminium‐housing. However, this reinforcement adversely increases the mass and volume of the battery, resulting in decreased specific energy, and it neutralizes the benefit of pouch cells of high specific energy.
Although both the resin‐ and metal‐housing‐based reinforcement were reportedly necessary for LIB pouch cells, we considered it was worth investigating whether LIC pouch cells work well in vacuum. To investigate vacuum tolerance of LIC pouch cells, we performed short‐term cycle life testing for two LIC pouch cells in a vacuum chamber. In the vacuum chamber, LIC pouch cells were placed on a thermostatic plate that was controlled to be 25°C by a coolant circulator (see Figure 8). As reference data, the same cycling test under a normal pressure condition was also carried out for other two LIC pouch cells. A single charge‐discharge cycle consists of 65‐min CP charging and 35‐min CC discharging with 80% DoD.
The pouch cells under normal pressure and vacuum are shown in Figure 8(a) and (b), respectively. The LIC pouch cells swelled similarly to the LIB pouch cells reported in . In spite of the significant shape deformation, the observed deterioration of cells in a vacuum was merely around 2%, compared to those under normal pressure, as shown in Figure 9. The capacitance retentions of cells dropped as the vacuum chamber was evacuated at the beginning of the testing (i.e., at an initial cycle). After this initial drop in capacitance retention, all cells exhibited the similar degradation trends. The measured capacitance retentions of cells in vacuum and under normal pressure at 5000th cycle (equivalent to 1‐year on‐orbit operation) were approximately 94 and 96%, respectively. Figure 10 shows the discharge curve trends of LICs during capacitance measurement. The dischargeable time gradually shortened as cells deteriorated. However, no significant voltage decline due to an increase in internal resistance was observed, indicating that the vacuum condition did not increase an internal resistance of LIC pouch cells. The results shown in Figures 9 and 10 suggested that bulky and heavy reinforcement using resin and/or metal‐housing would not be mandatory even though pouch cells swelled.
4.2. LIC pouch cell for NESSIE
NESSIE is a small demonstration platform, in which only one single LIC cell is allowed to be equipped. In general, for practical use, a plastic or metal container is used for pouch cells to be stacked and bundled to form a module. However, to reduce the mass of NESSIE, a container for the LIC pouch cell should be as simple and light as possible. In addition, although the results shown in Figures 9 and 10 implied that LIC cells might be used without reinforcement, the swelling should be avoided because it might cause unexpected interference with other components in NESSIE. To prevent the swelling at the lightest possible measure, we developed a dedicated metal‐bracket that looks ‘III‐shape’, as shown in Figure 11. This metal bracket can suppress the swelling at a light mass. This LIC pouch cell with the metal bracket successfully passed the vibration test. The specification of the LIC cell for NESSIE is shown in Table 3.
|Mass||303 g (bare cell)|
832 g (w/metal bracket)
|Dimension||123 × 165 × 15 mm (bare cell)|
164 × 220 × 23 mm (w/metal bracket)
|Manufacture||Asahikasei FDK energy device|
5. Development of NESSIE
5.1. Charge‐discharge regulator for LIC
The LIC cell is charged and discharged by a CDR that is basically a bidirectional dc‐dc converter. As explained in the previous section, LIC cells for the short‐term cycle life testing in the vacuum chamber were charged and discharged with the CP charging and CC discharging schemes, respectively. In practical use, the bidirectional dc‐dc converter plays the role of these charging and discharging schemes.
In general, a discharging power is simply determined by loads, and the bidirectional converter operates to regulate a load voltage at a constant value (e.g., 5 V in the NESSIE’s power system). On the other hand, a charging power is controlled by the bidirectional converter based on measured voltage and current. To this end, a feedback control loop including current and voltage sensors is necessary. Furthermore, the measured current and voltage values need to be multiplied to determine the charging power. In general, for small‐scale systems such as NESSIE, both circuit‐ and system‐level simplifications are of great importance to realize the miniaturized circuit and system. In other words, the current and voltage sensors as well as computational circuit for determining the charging power are desirably be eliminated from the system.
As a simplest possible solution, we employed a bidirectional buck‐boost converter operating in the discontinuous conduction mode (DCM), with which the charging power can be automatically constant even without feedback control nor calculation of product of current and voltage. The bidirectional buck‐boost converter and its key operation waveforms as well as current flow directions in DCM are shown in Figures 12 and 13, respectively. Switches Q1 and Q2 are driven so that an LIC is charged and discharged, respectively. The fundamental operation of the buck‐boost converter is well known and can be found in basic textbooks, and hence, this subsection focuses on the mechanism of the CP charging. One switching cycle contains three modes, and Toff_b period, during which no current flows in the converter, is unique to DCM operation; the inductor current iL reaches zero in the Toff_b period for every switching cycle. In this chapter, the operation for charging only is explained to save page length. To charge an LIC with the buck‐boost converter shown in Figure 12, the switch Q1 is driven while Q2 is always off. For discharging, on the other hand, the operation is vice versa—Q2 is driven, and Q1 is always off.
In the first mode, Ton period, iL increases linearly from zero, and its inclination is equal to Vin/L. At the end of this mode, iL reaches its peak value of Ipeak expressed as
where TS is the switching period and D is the duty cycle defined as Ton/TS.
As Q1 is turned off, the operation moves to Toff_a period, during which iL linearly decreases from Ipeak with the slope of –Vcell/L. iL starts flowing through diode D2 that is connected in parallel with Q2. The time length of this period, Toff_a, can be determined to be
As iL reaches zero, the final mode of Toff_b period begins. In this mode, except for the current of the smoothing capacitor Cout, no current flows in the converter.
In order for Toff_b period to exist, Toff_a must be shorter than TS - Ton, meaning Toff_a < TS - Ton. It leads to the operation criterion given by
The input current for the buck‐boost converter is supplied only during Ton period, and hence, the input power (or charging power) can be expressed as
This equation indicates that, with a constant on‐time of Ton, the charging power for the LIC cell or Pin for the buck‐boost converter can be automatically constant even without feedback control. In other words, open‐loop control is feasible, hence allowing simplified control circuit.
The photograph of a power control unit (PCU) that contains the buck‐boost converter is shown in Figure 14. The converter operated with a fixed D of 0.35 at a switching frequency of 100 kHz for the charging power to be approximately 3.8 W. For discharging periods, on the other hand, D was adjusted so that the load voltage was controlled to be 5.0 V.
The measured key operation waveforms during charging are shown in Figure 15(a). The inductor current iL swung as the gate signal for Q1 was applied. The measured iL was discontinuous triangular wave—the displayed iL is inverted—and the good agreement with the theoretical ones was observed. A single charge‐discharge cycling profile of an LIC cell with the buck‐boost converter operating in DCM is shown in Figure 15(b). The measured power during charging was nearly constant, verifying the automatic CP charging property.
The specification of NESSIE is listed in Table 4. The photographs of NESSIE installed in the main satellite HISAKI are shown in Figure 16. The high‐efficiency thin‐film solar cells were mounted on the main panel that was used as a lid for the interior components, including the LIC pouch cell and charge‐discharge regulator. NESSIE was installed in the side panel of the main satellite HISAKI.
|Dimension||550 × 463 × 205 mm|
|Bus||5‐V fully regulated|
|Power generation||>10 W (at end of life)|
6. On‐orbit operation
The main satellite HISAKI was launched by Epsilon‐1 rocket on September 14, 2013, and NESSIE was turned on for the first time on October 12, 2013, and the LIC was cycled in the first check‐up operation, as shown in Figure 17. During the first eclipse period, the LIC was nearly fully discharged to 2.2 V, followed by a CP charging during the subsequent sun period. The data at the beginning of the charging were temporarily not available because of an operation with low communication rate, with which essential housekeeping data for HISAKI only was acquired. The short‐term pulsating discharges during charging periods were due to the measurement operation for the thin‐film solar cells, during which the PV panel was short‐circuited and the LIC supplied power to the loads. The first check‐up results demonstrated the LIC as well as the charge‐discharge regulator performed well.
Since approximately 1 month after the first check‐up, the voltage of the PV panel of NESSIE has unexpectedly dropped. The decreased PV panel voltage was lower than the threshold voltage of the charge‐discharge regulator. In other words, the voltage of the PV panel was not high enough for the charge‐discharge regulator to operate, meaning the LIC has no longer been charged since then.
As the best effort we could do with the malfunctioning PV panel, we determined to turn on NESSIE only for a few minutes in every few months to observe the LIC pouch cell under the float condition. This is rather different from the originally planned cycling condition that was used for laboratory testing. But we consider that this floating operation of the LIC pouch cell would still be meaningful to demonstrate the on‐orbit vacuum tolerance and to investigate whether unexpected malfunctions would happen to the LIC after long‐term exposure to space environment.
The trend of the LIC voltage is shown in Figure 18. As aforementioned, NESSIE was turned on for a few minutes in every few months, and the LIC‐powered NESSIE and its voltage gradually decreased during the turn‐on periods. During the turn‐off periods, on the other hand, the voltage of the LIC unchanged as the voltages at the beginning and end of turn‐off periods were nearly identical, indicating the insignificant self‐discharge.
The on‐orbit LIC’s voltage trend shown in Figure 18 is redrawn as a function of discharged capacity in ampere‐hour and is compared to that of a cell tested in the laboratory, as shown in Figure 19. The on‐orbit and laboratory data matched very well, suggesting the LIC has been performing well.
Although the specific energy of LICs is far lower than that of traditional LIBs, the gap between LICs and LIBs can be greatly bridged once long‐cycle life performance with deep DoD conditions is factored in. In addition, LICs allow the CP charging scheme, with which the mass of the PV panel can be reduced compared to that with a traditional CC‐CV charging scheme. The mass of an LIC‐based power system was compared to that of a LIB‐based system. The results of the quantitative comparison suggested that, for applications requiring very long‐cycle life, for which LIBs need to be cycled with DoD shallower than 20% to achieve long‐cycle life, the LIC‐based power system would be comparable to the LIB‐based one, driving expectation that LIC would potentially be an alternative energy storage source.
We developed the technology demonstration platform named NESSIE whose one of the missions is the on‐orbit demonstration of the LIC pouch cell. Short‐term cycle life testing in vacuum was performed to investigate the vacuum tolerance of LIC pouch cells. The results of the 1‐year testing suggested the deterioration in capacitance retention due to the vacuum condition was insignificant. The dedicated charge‐discharge regulator, with which the LIC can be charged with the CP charging scheme even without feedback control, was also developed to realize simplified circuit.
NESSIE was launched with the main satellite HISAKI on September 14, 2013, and the first check‐up data showed the successful charge‐discharge cycling profiles of the LIC. However, the LIC has no longer been charged due to the malfunction of the PV panel since 1 month after the launch. Since then, as the best effort, the voltage trend of the LIC has been monitored to see whether long‐term exposure to space environment has negative influence on the LIC. As of this writing, no malfunctioning trend has been observed, suggesting the LIC has still been performing well.